A 100 metre-wide space rock known as 2001 EC16 paid a passing visit
to Earth’s vicinity earlier today. As it swept by at a little over
1.7 million km from Earth — approximately four and a half lunar
distances — the only people to pay it much attention were a dedicated
band of astronomers.

However, this will not always be the case. Although there was no danger
of a collision between the Earth and 2001 EC16, the day will surely come
when luck runs out for our world (and humanity). Our only chance of
survival is to detect the space invader long before a head-on collision
occurs. This is where two of ESA’s future missions — GAIA and
BepiColombo — may be able to play a vital role in forewarning us of
impending impacts.

GAIA will be ESA’s successor to the extremely successful Hipparcos
astrometry mission. As well as measuring the positions and brightness
of stars and galaxies with unprecedented precision, GAIA will be able
to detect all kinds of transient objects in its field of view —
supernovae, flaring stars and … asteroids.

“GAIA will detect objects brighter than magnitude 20, so we should
observe about one billion objects over the whole sky,” said Project
Scientist Michael Perryman. “This means that if there is something
there, we will see it. In the case of near-Earth asteroids (NEOs),
we should find objects as small as 500 metres in diameter.”

“The precision optics on GAIA will also give a colossal improvement
in orbital measurements, allowing astronomers to make very precise,
long-term orbit determinations,” he added. “This will allow them to
work out whether asteroids measured by GAIA will eventually collide
with Earth.”

“Although most of the large near-Earth objects will probably have been
found by the time of GAIA’s launch around 2010,” he explained, “the
NEO catalogue, compiled from ground observations, will not contain
all of the class of NEOs called Atens — asteroids that spend much of
their time inside Earth’s orbit. GAIA will be able to observe fairly
close to the Sun, so it will carry out a reasonably comprehensive
survey of these objects.”

Even GAIA will not be able to detect the entire population of asteroids
that cross Earth’s orbit and disappear in the glare of the Sun. However,
another ESA mission should make a significant contribution to the
ongoing census of potentially hazardous asteroids.

Although BepiColombo’s prime objective is to explore Mercury, it will
also be able to search for unknown asteroids in the uncharted region
of space between the planet nearest to the Sun and our Earth. These
NEOs are particularly dangerous, since they can approach the Earth
unseen against the brilliance of the Sun.

“The BepiColombo mission will involve two orbiters and a lander,”
explained Mission Scientist Rejean Grard. “One of the orbiters will
be used for planet-wide remote sensing and radio science on a polar
orbit with periapsis and apoapsis altitudes of 400 and 1500 km.
However we are also planning to have a telescope on this orbiter.”

“This telescope would be able to monitor a strip of sky of 6 degrees
by 360 degrees as it looks along the orbiter’s direction of motion,”
he added. “By detecting asteroids that cross this field of view and
comparing measurements made at different times, we could determine
their orbits. Preliminary evaluations indicate that there could be
up to 100 objects in the selected strip of sky at any one time.”

The Cosmic Shooting Gallery

2001 EC16 belongs to a growing family of space rocks larger than 100
meters across that can come closer to Earth than 0.05 AU (7.5 million
km). Fortunately, none of the known NEOs are presently on a collision
course with our planet, although astronomers are finding new ones
all the time. At the present time, 291 known potentially hazardous
asteroids have been detected.

How many NEOs are out there? No-one knows. Astronomers estimate that
there are between 750 and 1100 near-Earth asteroids bigger than 1
kilometre in diameter. There are probably millions of smaller objects
in orbits that carry them close to Earth.

Why does it matter? The amount of damage caused by an asteroid impact
depends on its size. Asteroids bigger than 1 kilometre would release
energy equivalent to 100 000 megatonnes of TNT — equivalent to 10
million times the power of the atomic bomb that flattened Hiroshima.
The result would be devastation on a global scale.

The good news is that such events happen on average only once every
300,000 years. The bad news is that collisions with medium-sized
objects are much more frequent — once in a few thousand years on

Even objects the size of 2001 EC16 pack a significant punch. In 1908,
an asteroid or comet just 50 metres across blew up in the atmosphere
above Siberia. If this explosion had occurred over central London,
the entire city would have been flattened.


* GAIA home page


* BepiColombo home page


* Space missions to asteroids


* Rosetta home page


* UK Task Force Report on NEO’s



[Image 1:
Artist’s concept of a catastrophic asteroid impact with the Earth. Life
near the impact would be instantly wiped out from the effects of high
temperatures and pressures. Injection of huge masses of dust (and gases)
into the atmosphere would effectively block out sunlight for long
periods of time to the point that most life could not be sustained
(“Nuclear Winter”). Images NASA (Don Davis).

[Image 2:
An exploded view of the GAIA satellite showing some of the system

[Image 3:
The BepiColombo spacecraft — ESA’s mission to the planet Mercury.